1. Technical Field
This invention relates to a method of generating inspection patterns for inspecting a patterned workpiece, e.g., for electron-beam inspection of optical photomasks.
It is well known that photomasks are used for exposing patterns onto a radiation sensitive layer formed on an integrated circuit during the fabrication thereof. The patterned photomask itself may be fabricated by selective irradiation with a focussed electron beam or other selective irradiation source. Light or other suitable radiation may then be projected through the photomask and onto a radiation sensitive layer on an integrated circuit, to produce an image of the photomask pattern in the radiation sensitive layer. In the current state of the art, photomask patterns for integrated circuit chips are becoming increasingly complex. Defects in the masks thus become significant contributors to yield and reliability losses. Accordingly, inspection of photomasks for defects is necessary.
2. Background Art
An inspection technique for optical photomasks using a direct write electron beam lithography tool is described in the publication "Detecting Submicron Pattern Defects on Optical Photomasks Using an Enhanced EL-3 Electron-Beam Lithography Tool" by R. A. Simpson et al., Proceedings of the International Society for Optical Engineering, Vol. 334, pages 230-237, 1982, the disclosure of which is incorporated herein by reference; and in U.S. Pat. No. 4,365,163 to Davis et al. entitled "Pattern Inspection Tool--Method and Apparatus", the disclosure of which is incorporated herein by reference. These references disclose a technique for deriving inspection patterns from the mask patterns themselves so that character or pattern recognition is not required to inspect the mask patterns. The inspection patterns may be generated by software from the original design data which was used to fabricate the mask patterns. As disclosed, in order to inspect a mask pattern, clear and opaque inspection patterns are required. Fixed-size overlapped electron beam spots are stepped over the mask as defined by the inspection pattern while a signal derived from the backscattered electrons is monitored to detect mask pattern defects. As pointed out, the clear and opaque inspection patterns are essentially shrunken forms of positive and negative versions of the mask patterns where the shrink provides a guard band that prevents mask and system tolerances from producing a false indication of pattern defects. As also pointed out, the inspection spots must overlap in order to provide proper defect detection probability.
The above mentioned dual requirements for inspection patterns, i.e., guard banding and overlapping, make it difficult to efficiently generate these inspection patterns from the mask pattern design data. More particularly, the mask pattern design data is typically expressed as a series of primitive shapes; e.g., a series of rectangles. When a mask pattern can be described in terms of isolated or non abutting primitive shapes, it is easy to provide the requisite guard banding by a well known process called windaging. A positive windage expands each dimension of a shape while a negative windage shrinks each dimension of a shape, thus providing the requisite guard banding outside or inside the primitive shape. However, if, as is typically the case, abutting primitive shapes are required to define a mask pattern, e.g., the mask pattern is a polygon which must be described as a series of abutting rectangles, then application of negative windage for guard banding purposes will separate the primitive shapes leaving a gap therebetween, and this gap will not be inspected.
In theory, a sorting technique may be devised to search for all abutted primitive shapes in the mask pattern design data, and then, after windaging, to somehow expand the primitive shapes to rejoin them. Such a search and expand technique would be extremely time-consuming in terms of computer time because it would involve a comparison of each primitive shape with every other primitive shape in order to search for abutting shapes. Such a search and expand technique also would have to provide an overlap between abutting primitive shapes in order to ensure the requisite inspection spot overlap where the primitive shapes abut.